Tube lamps have been preferred over incandescent lamps on account of their efficiency. A tube lamp such as a fluorescent tube lamp operates from a ballast, which regulates the current through the lamp. LED lamps are even more efficient than fluorescent lamps, and retrofit LED tube lamps have been developed. A retrofit LED tube lamp can be inserted into the housing of a fluorescent tube lamp fixture. Since the ballast is incorporated in the lighting fixture, the LED tube lamp must be able to operate from the “legacy” ballast. EP2793276A1 discloses the inductance of a variable inductance unit in the LED tube lamp is adjusted to make the magnitude of the current flowing through the LED unit be stabilized so as to fall within the predetermined range. US20140225519A1 discloses a detection circuit that detects the operating frequency of the ballast so as to differentiate electronic ballast or inductive ballast, and a LC arrangement is configured according to the detected operating frequency.
There are many different high-frequency ballasts. One widely used design is the half-bridge LC resonant circuit. The control circuit of such a ballast is usually one of two major types: self-oscillating using a ring core, or IC-controlled. The self-oscillating circuit is generally a “low-end” and economical solution, while the IC-controlled circuit is “high-end” and generally more expensive.
However, the different control circuits described above are characterised by difference responses when an LED tube lamp is connected to the ballast. In the case of an IC-controlled ballast, which has a good current source characteristic, the ballast output current or “lamp current” does not vary much when LED tube is connected. However a self-oscillating ballast has a soft current source characteristic, with the result that the ballast output current increases when an LED tube is inserted into the lighting fixture. This increased current causes severe reliability problems for the self-oscillating ballast due to overheating of the ballast internal circuit components. In some cases, a self-oscillating ballast can fail shortly after the LED tube lamp is inserted and switched on.
However, a customer wishing to replace a fluorescent tube lamp by a corresponding LED tube lamp may not be aware of this issue, and may not be able to determine whether his lighting fixtures will operate reliably with the known types of LED tube lamp available on the market. Since self-oscillating ballasts are widely used in certain regions such as the APR (Asia Pacific Region) market and part of the EMEA (Europe, Middle East and Africa) market, problems with compatibility are to be expected for the existing LED tube lamps.
Therefore, it is an object of the invention to provide an improved LED tube lamp that avoids the problems mentioned above.